In the event of a terrorist attack using radioactive material, risk-stratification of patients based on exposure doses will be critical to ensure appropriate care. Current dosimetry assays take days to perform, which would delay life-saving therapies. This proposal seeks to develop RNA-based dosimetry assays using the GeneXpert platform that will risk-stratify patients immediately after radiation exposures. Specific Aim 1. Discover genes that display dose-dependent expression changes after in vitro irradiation of human peripheral blood cells. Radiation exposures cause dose-dependent expression changes in peripheral blood cells. We hypothesize that these expression changes can be utilized to develop RNA-based assays that discriminate between clinically relevant doses of irradiation. Therefore, DNA microarrays will be used to identify genes with dose-dependent expression changes in hematopoietic cells after in vitro irradiation. Quantitative RT/PCR assays will validate these dose-dependent expression changes, and candidate genes will be examined in Specific Aims 2 and 3. Specific Aim 2. Determine whether RNA-based dosimetry assays using plasma or cells best correlate with radiation doses in the canine model. Cells injured from radiation die, leaking RNA into the extracellular space, and this RNA can be detected using quantitative RT/PCR assays. We hypothesize that RNA-based assays using plasma will be more specific in discriminating between radiation doses. Ethical issues preclude systematic examinations of radiation responses in humans, but dogs offer an optimal animal model to examine the in vivo effects of irradiation. Therefore, quantitative RT/PCR assays for candidate genes from Specific Aim 1 will be examined in the canine model to determine which genes display in vivo dose-dependent expression changes and whether plasma or blood cells should be used for the development of dosimetry assays. Dosimetry assays will then be developed using the GeneXpert platform, and these GeneXpert dosimetry assays will be validated in additional dogs. Specific Aim 3. Determine if in vitro dose-dependent genes correlate with radiation exposure in transplant patients. There may be subtle differences in radiation-induced responses between dogs and humans. In addition, exposure over more extended periods of time may invoke different radiation responses. We hypothesize that RNA expression changes may be useful in determining the cumulative dose of irradiation after prolonged exposures. Therefore, quantitative RT/PCR assays for candidate genes from Specific Aim 1 will be examined in transplant patients in order to determine which candidate genes display dose-dependent responses in humans receiving irradiation over several days and whether plasma or blood cells are optimal for the development of dosimetry assays in humans.